The Communication increasing via electronic devices demanded high standard ways to ensure the information confidentiality through cryptography algorithms. At the same time, several studies have been made in attempt to break that confidentiality, either by obtaining the knowledge of the plaintext without the knowledge of the cryptographic key or with the knowledge of the key itself. In this paper, we propose a method capable of clustering cipher texts by cryptographic keys, based on clustering techniques. In the clustering experiments on a set of encrypted texts by the algorithms AES(keys of 128, 192, 256 bits), DES (key 64 bits) and RSA (keys of 512 and 1024 bits) success was achieved since all ciphertexts encrypted with the same key belong to the same group. This result puts into question the randomness proposed by cryptographic algorithms, that are designed to generate cipher texts without any relationship with the input data, such as the legible text and the cryptographic key.